The fz5 adapter has 55mm threads, so any 55mm add-on lens will fit it directly. (Of course that doesn't guarantee compatibility.) However, the threads are recessed about 1mm into the front of the adapter and that can interfere with a secure threaded connection, thus the recommended sanding.52mm add-ons need a 55-52mm step down ring and work just as well as 55mm add-ons though in some cases (ex, wide angle lens) the added distance from the camera lens can cause problems like vignetting.